Platinum‐group element and stable isotope geochemistry of PGM‐bearing troctolitic rocks of the Duluth Complex, Minnesota

Abstract

Platinum‐group minerals (PGM), selenides, arsenides, graphite and the common association silicates + sulphides + oxides were identified in the basal troctolites of the Duluth Complex. Platinum‐group element (PGE) concentrations (ppb) in the troctolites are as follows: Os (3–15), Ir (0.1–25.1), Ru (1–50), Re (1–13), Pt (25–390), Pd (5–1300). Element ratios (Pt+Pd/Ir+Ru+Os ranging from 2.66 to 61.77 and Pd/Ir from 16 to 245) are in the range of flood basalt related deposits. The PGE/chondrite distribution is similar to most mafic‐ultramafic rocks. The overall PGE data define two distinct PGE‐chondrite normalized distributions. δ34S and δ18O data range from +4 to +9 ppm and +2 to +7.3 ppm respectively. A maximum δ34S value of +13 ppm has been documented in a graphite‐rich troctolite. The δ34S data lie within a narrow range of values different from those reported for the Cu‐Ni mineralized troctolites (without PGM) and similar to the PGE‐rich Babbitt zone. This suggests a different mechanism for the origin of the PGM‐bearing rocks in comparison with the Cu‐Ni mineralized zone. The low δ18O values (<5) are from magnetite‐rich troctolitic samples. Most of the platinum‐group minerals are associated with hydrous silicates (prehnite, actinolite, chlorite, biotite, serpentine) and secondary sulphides (valleriite, violarite). Altered silicates in the mineralized samples contain high amounts of Cl (e.g. 0.5–1.1 wt% Cl in serpentine).